Process of polymerization



Patented Jul -23,1935 h 2,003,719

i rnooass or.PoLr uEnIzA'rIoN e I 'I Glen M. 'Ku'ettel, Roselle, N. J.,' assignorto Du .uz-fiPont' ViscoloidsCompany, Wilmington,- Del.,- a

corporation of Delaware [[No Drawine- Application July 24,1934, t

V j f s rial-Nausea s I I i 8,Claims. (Cl. 260.-2)' t v 'Ihisfinvention relates to a processor polymerirate. Usuallylthe mass being polymerized is held zation and, 'more particularly; to animproved [at aftempe'rature of about 80C. in this process. a process of polymerizing the liquidpolymerizable By the termtempe rature, of bubble .formation compounds of the 'class including acrylic'jand 2.1- as usedherein is meant thattemperature at which 7 kacrylic acids, their 'homologues, and derivatives, some component of the masstbeing' polymerized 5 to a bubble free solidflhaving no internal visual U islvolatilized ordissolved air, or" the like is redefects; The invention particularly relates to leased in the. mass' This temperature is not such processes in" which liquid "methyl (alpha necessarily. as high as the boiling point of the methacrylatelmonomer is polymerized, I monomer-"being treated since byproducts of the -10] 'Methyl alphamethacrylate monomer'is a, liquid polymerization, or other components, if any, of 10 having aboiling pointoflOO -lOllC.,}which. may tne mass may be volatilized at a temperature be readily polymerized to a solid. 'Its preparalower than the boilingpoint; of the monomer, or tion is disclosed in Rowland Hill QULSZ' applicadissolved gases may bereleased at a temperature 'tion Serial No. 64 1 ,'1'l3, filed November "3,'f1932. lower than the boiling point of the monomer, in This liquid, as w ell as the: otherliquid polymerizeither case causing the formation of bubbles, pos- 15 able compounds of the classmfention ed above, =sibly'. at a temperature appreciably below that at may bepolymerized in meek or other solidshapes which the monomeritself would volatilize. free from flaws, ifa relatively longv period of while the processdisclosed in thevLoderaptime is allowed for the resin to set up orpolymerplication is believed to be theoretically correct to ize, for example, 14-30 days. Such aftirneconeffect'rapidlysubstantially complete polymerizago Suming procedure increasesinordinately the cost tion of a mass without formation of internal offthe finished product,"di1e to the. tie5upjof apflawssuch as bubbles, and in solid masses of paratus and of material in process] j' 1' relatively small size may be carried outto ex- By employing polymerization atalysts, -high cellent advantage, practicaloperating diificulties temperatures, a'ctinic li ht, and/or other'nieans, in control of temp ratur are n unt d where 26 it is known that' polymrization may be greatly l e'mflss in solidtshapesi 0f m mer' speeded up. However; where the polymerization e n p meriz d. Wh reas smaller masses of has been materially accelerated by heretofore ';m ay be expeditiously p y e d w methods, pecially n polymerizingim according to the Loder process without danger 30 'terial in solidshapes, the" resulting product-has 9 bubble forma and internal flaws n, the 30 always contained internal bubbles, flaws, and fin D du0t, yet, when r ass s of the other visual defectswhichrendersit mdro'or less m n mer e e n me it is necessary, unfit for fabrication into articles which are valt w 1 th dangeriof bubble r ati t0 p 1 'uable' because of their L appearance. Moreover, the averagee perature of the a qu low s thelarger the sizeoithesolid Shapathe'greater thatlocalportions Of themass may not heat 35 'thet ytofo mTbobbles n ia o h fere u i t m e a u b b forlnflitiml-v eer the tendencyfor incomplete?polymerization QWSQiFiS- S a y 0 keepv the av ag t of the whole; for,"although the cuterfsurface of p r of t e m ss e v ly l w durin th thmas s may be completely polymerized; thejcen o mer on e s ed of polym n is 40 ter is often only partially'polymerized andfin 'i l l we eup' e compared 40 someinstances, substantially not atau. ,7 i th speed at [which thelpolrmerizetionmey VIII U; 5; appl cati n Serial No. 72 8;550jof Dontake place in smaller masses .of the monomer. ald J fLoder,filedJune 1, 1934, entitled Process An je o t present invention is to p of polymerization, is disclosed an. improved Vid an D S OVel the above 1 method of polymerizing to avoidthe "objections ed Lod p c in -p rizine'com 45 of the prior art, as discussed above. This process Mons C0ntai11ing liquid y alpha methacomprises essentially heating thejliquidjrnethyl yl te m om r and a po p alpha methaerylate monomer to a, oimfl me ticularly in larger masses and solid shapes. A whatbelow the temperature of bubble forma tion further object is to provide aprocess in which to initiate polymerization andthen withdrawing t practical dilficullties in C ll h 13em- 50 i th xothermi he t resulti g f m thepo'lymeriperatureof the mass during polymerization are zation reaction at"substantiallyflfthe rate it' 'is OVBICO A fu th bj t 05 t nvention is formed; In this manner, formation ofw'bubbles to provide a practical and economical process of 1 and other'internal flaws is avoided and yet polypolymerizing thesecompounds in large masses to merization ma'y be carried out ata relatively fast give a bubble free final product. Other objects 55 of the invention will be apparent from the description given hereinafter.

The above objects are accomplished according to the present invention by heating a continuous mass comprising a liquid polymerizable compound from the class consisting of acrylic and alkacrylic acids, their homologues, and derivatives, until the temperature of at least some portion of the mass approaches the temperature of bubble formation, cooling said mass down until the polymerization reaction caused by the heating is substantially stopped throughout the mass, preferably the mass is cooled down until no portion of the same has a temperature substantially in excess of room temperature, again heating said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, and then again cooling the mass as before. In some instances, at the end of this second heating treatment, the mass will be substantially completely polymerized. However, if it is not, the last two steps are repeated until the mass is substantially completely polymerized.

Among the compounds suitable for use in this invention may be mentioned acrylic acid and alkacrylic acid such as methacrylic and ethacrylic acids, their esters, derivatives, and homologues such as the nitriles, amides, substituted alkyl and aryl amides of these acids, as well as, more particularly, the esters of the alkacrylic acids as the alkyl esters of methacrylic and ethacrylic acids. Specifically may be mentioned monomeric methyl alpha methacrylate, ethyl alpha methacrylate, methyl alpha ethacrylate, and ethyl alpha ethacrylate. This class of compounds has the common characteristic of being liquids in monomeric form at room temperature and atmospheric pressure and polymerizing to solid thermoplastic resins. The present invention is also applicable to mixtures and interpolymers of these compounds.

In polymerizing one of these liquid monomeric compounds it is first subjected, in any suitable container, to a period of heating in order to initiate polymerization. The polymerization reaction is an exothermic one and, if it be allowed to proceed without control, particularly in a relatively large mass, bubbles will be formed by voltatilization of some component of the mixture or the releasing of dissolved air, or other gases. While the mass is in a relatively liquid state, such bubbles will promptly rise and leave the mass but as the material gradually thickens during polymerization, passing into the condition of a viscous liquid or a solid, the escape of bubbles ultimately becomes impossible, with the result that the polymerized mass in final form contains bubbles. It is necessary, therefore, that, once the liquid has reached a viscous condition, the reaction must be controlled in such a. way that no bubbles can be formed. The formation of bubbles in the very early stages of the polymerization is not such a serious matter.

According to the present invention, heat is applied to the monomeric compond until some portion of the mass approaches the temperature of bubble formation. When this point has been reached the mass is then cooled down until the polymerization reaction is substantially stopped, preferably the mass is cooled down until it does not exceed room temperature in any portion thereof, Thereupon the mass is again heated until the temperature of bubble formation is approached, the cooling step again being applied,

and the alternate heating and cooling continued until a substantially completely polymerized mass is obtained.

In order to operate the invention successfully it is, of course, necessary that in none of the heating steps, except at the beginning where the mass is in a very liquid form, that the temperature in any portion of the mass be allowed to reach the temperature of bubble formation. On the other hand, in order to operate the process as expeditiously as possible, the heating should be carried out just as long and to as high a point as may be safely done without formation of bubbles. The temperature to which the mass may be safely heated without danger of bubble formation is determined by the specific composition to be treated, the size of the mass, and the shape of the container. Therefore, in commercial operations a sample batch of the exact composition to be polymerized, in the exact amount, and in the same shaped container, is heated continuously until formation of bubbles is effected. This temperature is noted and, in carrying out the polymerization of subsequent batches, the heating of the mass is regulated so that the temperature in no portion of the mass quite approaches the temperature of bubble formation, as determined in the sample batch. In so operating, polymerization may be carried out as fast as possible with a determined degree of safety. Likewise, the duration and extent of the cooling period can also be determined upon a sample batch of the same composition, in the same amount, and in the same shaped container. The ideal operation of the process would mean that the mass is heated to as high a temperature and for as long a period as possible without the formation of bubbles and would be cooled down as little as possible while still substantially completely stopping the polymerization.

The purpose of the cooling step is to enable control of the operating conditions at relatively high temperature. These compounds have poor heat conductivity and, therefore, since the polymerization reaction is exothermic, the building up of excessive temperatures in local portions of large masses of the mass is unavoidable. Usually the highest temperature is approximately in the center of the mass. By holding the mass at a definite temperature slightly below the temperature of bubble fo'rmation, it would appear that polymerization of the mass could be effected in the shortest time. This is true in polymerizing smaller masses but in a large mass it is not true because, in order to prevent any local portion of'the mass rising to the temperature of bubble formation, it is necessary to maintain the average temperature of the mass so low that polymerization is rather slow, even though means such as a water bath are employed to withdraw the exothermic heat of reaction at substantially the rate it is formed; the poor heat conductivity of the mass makes it impossible, in handling large masses of material, to effectively control the withdrawal of the heat.

According to the present invention, it has been found that the mass may be heated up to a higher temperature for a short period without danger of any portion of the mass reaching the temperature of bubble formation. Then, by cooling the mass down so that polymerization is substantially stopped and locally overheated portions are cooled oil, the temperature can then again safely be raised to a relatively high point for another heating period without danger of formation of bubbles. By operating in this manner with alternate heating and cooling, it is possible to polymerize large masses of these compounds in sol-id shapes without danger of bubble formation ina shorter total period than if the mass were heated throughout to a temperature low enough to safely avoid bubble formation.

In order to illustrate the present invention, the following examples are given:

Example 1.Liquid methyl alpha methacrylate monomer is poured into a lead mold of inside diameter 0.75" and of length 20", which mold is placed in a vertical position in an oven maintained at from 80-84 C. and is held therein for 3 hours.

At the end of this period the temperature in local portions of the mass will have been found to have risen to a point close to the temperature of bubble formation. The mold is then removed from the oven and allowed to cool to approximately room temperature. Three hours after its removal from the oven it will be found that the mass throughout is cooled in all portions thereof and it is then again returned to the oven for a second period of three hours at the same oven temperature. This procedure, upon being repeated for a total of five complete heating and cooling stages, will give a substantially completely polymerized resin which, upon removal from the mold, will be an exceptionally clear rod free of bubbles and other internal flaws.

Example 2.A mold having a cavity in the form of a tube of outside diameter 3.5" and inside diameter 3.0" and height 6", was filled with liquid methyl alpha methacrylate monomer and placed for 3 hours in an oven at a temperature of C. It is then removed and cooled to room temperature for 3 hours. It is then alternately heated for 2 hours and cooled for 3 hours at the same respective temperature, for '7 additional cycles. The tube of methyl alpha methacrylate polymer removed from the mold is crystal clear and free from bubbles and other internal fiaws.

Example 3.Ethyl alpha methacrylate monomer is poured into a lead mold identical with that used in Example 1 and the same periods of heating at the same temperatures and the same periods of cooling at room temperature are carried out for a total of 5 complete heating and cooling stages. The resin is then removed from the mold as a clear rod free of bubbles or other internal flaws.

It will be understood that the above examples are merely illustrative and that the specific temperatures and times for heating and cooling will be varied according to the particular composition employed, the amount of said composition, and the shape of the mold. Whereas the invention is broadly applicable to the compounds of the class herein described and the operation of the invention will be readily understood by those;

skilled in the art, it is necessary, in order to establish the optimum periods of heating and cooling and the respective temperatures used, to make a trial with a specimen batch under identical conditions with those to be used in the commercial operation of the process. Ordinarily in handling large solid shapes of vmethyl alpha methacrylate resin, the heating would be carried out in a water bath or oven or equivalent heating means held at a temperature of 70-85 C. and the heating period would be in the neighborhood of 3 hours. However, even for this specific compound the temperature of heating and the duration of the heating may be widely varied under different circumstances, which can only be determined for anything approaching the optimum conditions by trial on a specimen bath under identical conditions. Likewise, where others of this class of compounds are employed, a trial run on a specimen batch would also be necessary to determine optimum conditions. Y

The alternate heating and cooling treatments may be conducted in any convenient apparatus and, on a commercial scale, it is usually convenient to work out the heating and cooling periods in such a way as to effect economical use of the equipment, for example so as to permit material being polymerized to be interchanged between the heating and cooling equipment so that both may be kept in substantially constant use. The cooling may be effected by artificial means if desired, it being obvious that the period of cooling can be shortened if the mass is exposed to an extremely low temperature. It is to be understood that; in connection with the cooling, it is necessary that the mass be cooled down sufficiently so that upon commencing the re-heating no portion of the mass will be appreciably warmer than any other portion, otherwise the heating cycle will have to be terminated in a short while because some local portion of the material has approached the temperature of bubble formation, whereas the greater part of the mass is at a relatively low temperature. It will be apparent that the higher the temperature applied in the heating stage, the shorter must be the duration of that stage because of the fact that at least some porion of the mass will more quickly reach a temperature approaching that of bubble formation.

The process may be carried out with. vessels or molds of any shape or material. In accordance I with common practice a cast lead mold having a slight taper is satisfactory, but glass vessels may be used, as well as vessels or molds of other materials which do not have specific unfavorable influences upon the progress of the polymerization reaction. Ordinarily a tapered mold will be employed to admit the release of the finished mass but split molds may likewise be employed. If desired, the polymerization may be carried out under positive pressure.

For the sake of simplicity the invention has been described with relation to the polymerization of a single compound coming within the class of compounds to which this invention relates. However, the operation of the invention is the same whether one or a mixture of these compounds are employed. Likewise, various auxiliary agents may be added to the monomer prior to the polymerization process. Such agents would include accelerating agents, plasticizers, coloring matter, effect materials, and the like. In the Hill application previously referred to a large number of the suitable plasticizers, accelerators, and other materials which may be added, are disclosed. Where a plasticizer is employed in relatively high amounts, for example, 40% by weight of the compound being polymerized, the heat conductivity of the mass is materially improved and the difficulty in constantly maintaining a temperature approaching the temperature of bubble formation is therefore decreased and, in smaller masses, fair speed of polymerization may be obtained without the cooling step of the present invention. However, where a larger solid mass is to be polymerized, the difiiculty in controlling the temperature throughout the mass is encountered the present invention may be applied to the same advantage as where a smaller proportion of plasticizer, or no plasticizer, is being used.

The present invention provides a method of preparing glass clear resin produbtsfree of visual flaws, in large masses, without encountering conditions substantially impossible of control in commercial operation. Objects of any section,

contour, and thickness, whether solid or hollow,

may be made, as for example, in the manufacture of slabs, blocks, rods, tubes, rings, electric insulating parts, or any other articles such as are commonly produced from phenolic resins, cellulose derivatives, and like materials. Resinous bodies may be prepared to be fabricated into articles by machining, or otherwise cutting, from rods, blocks, and the like, for the preparation of a wide assortment of objects such as beads, bracelets, and the like. The compositions may be made transparent, opaquejplain or colored, and with any desired pattern or mottle, as for instance imitation pearl, tortoise shell, and the like.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

I claim:

1. Process of preparing a bubble free product comprising heating a continuous mass comprising a liquid polymerizable compound from the class consisting of acrylic and alkacrylic acids, their homologues, and derivatives, until the temperature of at least some portion of the mass approaches the temperature of bubble formation, cooling said mass down until the polymerization reaction caused by the heating is substantially stopped throughout the mass, again heating said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, and again cooling the mass as before.

2. Process of preparing a bubble free product comprising heating a continuous mass comprising a liquid polymerizable compound from the class consisting of acrylic and alkacrylic acids, their homologues, and derivatives, until the temperature of at least some portion of the mass approaches the temperature of bubble formation, cooling said mass down until no portion of the mass has a temperature substantially in excess of room temperature, again heating said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, and again cooling the mass as before.

3. Process of preparing a bubble free product comprising heating a continuous mass comprising a. liquid polymerizable compound from the class consisting of acrylic and alkacrylic acids, their homologues, and derivatives, until the temperature of at least some portion of the mass approaches the temperature of bubble formation,

. cooling said mass down until the polymerization reaction caused by the heating is substantially stopped throughout the mass, again heating said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, again cooling the mass as before, and repeating said last two steps until the mass is substantially completely polymerized.

4. Process of preparing a bubble free product comprising heating a continuous mass comprising a liquid polymerizable compound from the class consisting of acrylic and alkacrylic acids, their homologues, and derivatives, until the temperature of at least some portion of the mass approaches the temperature of bubble formation, cooling said mass down until no portion of the mass has a temperature substantially in excess of room temperature, again heating said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, again cooling the mass as before, and repeating said last two steps until the mass is substantially completely polymerized.

5. Process of preparing a bubble free product comprising heating a continuous mass comprising liquid methyl alpha methacrylate monomer until the temperature of at least some portion of the mass approaches the temperature of bubble formation, cooling said mass down until the polymerization reaction caused by the heating is sub stantially stopped throughout the mass, again heating said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, and again cooling the mass as before.

6. Process of preparing a bubble free product comprising heating a continuous mass comprising liquid methyl alpha methacrylate monomer until the temperature of at least some portion of the mass approaches the temperature of bubble formation, cooling said mass down until no portion of the mass has a temperature substantially in excess of room temperature, again heat ing said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, and again cooling the mass as before.

7. Process of preparing a bubble freeproduct comprising heating a continuous mass comprising liquid methyl alpha methacrylate monomer until the temperature of at least some portion of the mass approaches the temperature of bubble formation, cooling said mass down until the polymerization reaction caused by the heating is substantially stopped throughout the mass, again heating said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, again cooling the mass as before,-and repeating said last two steps until the mass is substantially completely polymerized.

8. Process of preparing a bubble free product comprising heating a. continuous mass comprising liquid methyl alpha methacrylate monomer until the temperature of at least some portion of the mass approaches the temperature of bubble formation, cooling said mass down until no portion of the mass has a temperature substantially in excess of room temperature, again heating said mass until the temperature of at least some portion of the mass approaches the temperature of bubble formation, again cooling the mass as before, and repeating said last two steps until the mass is substantially completely polymerized.

GLEN M. KUETTEL. 

